The present invention relates to a method of manufacturing units containing sorbent material. Such units may be intended for use as thermal accumulator elements for a heating or refrigeration system. The invention also relates to units manufactured by such a method.
It has been long known to use materials that are known as "sorbents" especially in connection with refrigeration. In certain refrigeration processes a sorbent is utilised which consists of a solid substance that absorbs a sorption medium in gaseous form as a reversible exothermic reaction. For example calcium chloride may be used as a sorbent with water as sorption medium. In a slightly different case the refrigeration process is effected by means of an adsorption. An example of this method is the use of zeolite as a sorbent and water vapour as a sorption medium. In both these cases a solid substance is used as a sorbent, and the process is reversible. Thus initially the sorption occurs with the development of heat. Subsequently this process is effectively reversed and the sorption medium is removed from the sorbent by supplying heat.
It has been proposed that the same or similar sorption processes can be used for other purposes, for example for storing heat derived from solar energy. An example of a system that has a heat storage capability is a chemical thermal pump which consists of two communicating vessels, one vessel containing a sorbent on which the sorption medium is adsorbed or absorbed. By a suitable selection of sorbent and sorption medium a cyclic process can be achieved. The first mentioned vessel with the sorbent, is supplied with heat, for example by contacting the vessel with warm water from a solar energy collector. As heat is supplied, in an endothermic process the sorption medium is evaporated off from the sorbent in gaseous form and subsequently condenses in the second vessel while emitting a certain amount of heat. When all the sorption medium has been evaporated from the sorbent the thermal pump can be considered to be a thermal accumulator which is fully charged. During the next cycle the condensate in the second vessel is caused to vaporize by supplying what is known as "low-temperature" heat and is sorbed into the sorbent, thus giving off substantial quantities of heat at a higher temperature than the "low-temperature" heat.
Many different combinations of sorbents and sorption medium have been suggested for heat storage devices that operate in this, or a similar manner. In most cases substances have been suggested for the sorbent which are in solid form during most of the process. Examples of such substances are various salt hydrates, in which case the sorption medium is water vapour; ammoniates, in which case the sorption medium is ammonium; and zeolites, in which can the sorption medium can be water.
The general conditions for commercial suitability of a sorption process, whether for refrigeration or heat storage, for instance, are amongst other things, that the sorbent and sorption medium are both inexpensive, not damaging if released to the environment and capable of undergoing repeated cycles of use without deteriorating in any way. Partly for these reasons the greatest interest in heat storage has been associated with salt hydrates with water as sorption medium, but other combinations have been tested.
In most cases the sorption processes requires the substantial absence of air if the process is to be sufficiently quick to be useful. For both refrigeration and the storage of heat for example, in the original form of solar energy, this generally means that the processes must take place in a partial vacuum in relation to atmospheric pressure. If, for instance, when using a chemical heat pump which employs a salt hydrate and water, it is desired to make use of "low-temperature" heat which may have a temperature of about 5.degree. C., in order to evaporate water which is then to be sorbed by a salt hydrate, for instance, which will then develop heat at a temperature of perhaps 40.degree.-60.degree. C., the pressure in the system (i.e. the partial pressure of the water vapour) will have to be between perhaps 6 and 10 torr. Corresponding ammoniate systems and zeolite systems also usually require low pressure.
When storing heat by the week or seasonally in order to utilize solar energy in a practical manner, by a sorbent method the quantity of sorbent needed will be considerable. A theoretical calculation indicates, for instance, that in order to store 7000 kWh, using calcium chloride as sorbent and water vapour as sorption medium, 15 tons of the salt would be needed if practical extraction temperatures are to be used. Such a quantity of salt would itself occupy about 20 m.sup.3.
However, since heat is to be supplied to and removed from the sorbent, in all practicable systems one or more heat-exchangers must in some way be incorporated into the sorbent layer used to form a heat accumulator. This means, therefore, that the actual volume of the thermal accumulator will be considerably greater than the volume of the salt alone. Considering merely the volume of salt specified above it can be seen that this leads to a major problem since in a practicable system it must be possible to supply heat to the heat accumulator to remove it therefrom in an economical and practical manner.
Since the sorption process generally requires the absence of foreign gases such as air, considerable problems arise with respect to supplying or removing heat to and from the sorbent in an economic manner without the overall volume of the thermal accumulator becoming too great. The problem becomes greater in proportion to the quantity of sorbent.
Another problem is that the sorbent material generally has relatively low thermal conductivity and that the total heat-transfer surface of the heat-exchanger must therefore be large in order to permit sufficient output both when storing and when extracting heat. If the heat accumulator is designed so that the sorbent is placed in one or more tanks in which a heat exchanger is located, so that heat can be supplied or extracted, there will be considerable stress in the tank walls caused by the pressure difference between the interior of the tank and the ambient atmosphere. The walls of the tank must therefore be dimensioned to withstand extremely high pressure differentials without rupturing which leads to enormous costs.
A two-tank system has already been proposed using a salt hydrate as sorbent and water as sorption medium, the sorbent tank consisting of a multitude of steel tubes filled with the hydrate. The tubes are connected together and also to the sorption tank containing water. This tube system, which is under greater reduced pressure, is immersed in a tank container a thermal transfer medium such as water. Upon discharging of this two tank thermal accumulator, water is sorbed by the salt and thus the salt is heated. The heat is conducted through the salt, out through the tube walls to the surrounding water. Thus in this case the sorbent tank with its heat exchanger consists of a tube system surrounded by an outer tank.
A simple calculation shows that even in this case a heat accumulator of this type would be enormously expensive. There are several reasons for this. One is that the tubes must be dimensioned to withstand the external pressure, having regard to the fact that the interiors of the tubes are at a very low pressure. Another is that a considerable length of tubing is required to ensure that all parts of the salt are sufficiently accessible for heat transfer. A third is that the material in the tube walls must have good thermal conductivity since they must be relatively thick in order to withstand the external pressure and yet heat must be readily able to pass through the tube walls. A rough estimate indicates that tubes having sufficiently thick walls to withstand the external pressure and having a sufficient expanse of heat-transfer surfaces with good thermal conductivity, made of steel for instance, would be very costly (several hundred thousand Swedish crowns) in material costs alone for seasonal storage of heat for a one-family house. A problem which is also inherent in the thermal accumulators described above is that, as known, hydrates swell and shrink during the cyclic sorbtion and de-sorbtion process. This means that difficulties are incurred in obtaining satisfactory thermal contact between the salt and the walls of the tubes, since spaces are often caused by the movement of the salt, particularly between salt and tube walls. Furthermore, if the tubes are packed tightly with the sorbent material the gas to be absorbed may be impeded in its flow due to the salt swelling.